QC vs. Clinical Limits

September 10, 2014


This entry adds to but does not contradict the Westgard Web site’s: http://www.westgard.com/badqc-goodsoftware.htm

Clinical limits provide a range of results that do not cause patient harm.

A problem with traditional clinical limits is that going from just under the limit to just over the limit changes the outcome from no harm to harm. This is difficult to understand for two results that are almost identical. That is why error grids were developed. They separate harm into categories so when one goes from just under the limit to just over the limit the outcome changes from no harm to minor harm.

QC limits are set to control the process

Thus, QC and clinical limits are different.

For an assay with a high process capability (similar to a high six sigma) clinical limits will always be much wider than QC limits. And for an assay that is out of control (values that exceed QC limits), results may still be within clinical limits. But it is nevertheless important to detect an out of control process because the process may become so out of control so that results fail clinical limits.

For a process that is in control (all values that within QC limits), can results be outside of clinical limits (potentially cause patient harm?) YES!

Glucose examples:

Patient spills coke on himself. Provider comes in to use glucose meter and fails to wash and dry the site from which the capillary sample is taken. The result is 200 mg/dL too high. There is nothing wrong with the glucose meter or QC.

Patient has an interference. Result is way off. QC is ok.

There is a one hour shift in results – they are way off. QC – once every eight hours doesn’t detect the shift.

Thus, QC detects long-term bias which is important but there can still be other errors that can harm patients.